In radio communications, signals are sometimes lost or impaired due to a variety of phenomena such as multipath fading, time dispersion, and co-channel interference which exist in a typical radio communication channel. Multipath fading results from the interaction of the transmitted signal and its reflections or echoes which arrive at the receiver at approximately the same time. If the number of reflections is relatively large, this fading exhibits a so-called Rayleigh distribution. Time dispersion occurs when there is a time delay between the reflections and the transmitted signal. Interference results from the presence of signals which are non-orthogonal with respect to the transmitted signal. Such non-orthogonal signals can originate from other radios operating on the same frequency (co-channel interference) or from other radios operating on neighboring frequency bands (adjacent channel interference).
FIG. 1 shows an example of co-channel interference, in which a mobile station M1 communicates with a base station antenna A1 in a cell C1, while a mobile station M2 communicates with a base station antenna A2 in a cell C2. A base station antenna A3 serving a cell C3 may be located between cells C1 and C2. In this example, mobile stations M1 and M2 are simultaneously communicating on the same channel to different antennas in different cells. As shown, signals transmitted by mobile station M1 to antenna A1 interfere with the signals transmitted by mobile station M2 to antenna A2, causing signal impairment.
To reduce the effects of such signal impairments, it is known to use diversity combining, in which a receiver is provided with multiple separated antennas, and the received signals at each of the antennas are combined. Because the antennas are separated, the signal strength in each antenna is independent. Thus, if there is a deep fading dip for one antenna, another antennas may have a relatively strong signal. There are many types of diversity combining methods. For example, in Mobile Communication Design Fundamentals, by William C. Y. Lee (Wiley, 1993), numerous diversity schemes are described at pages 116-132.
In a typical mobile communication system, antenna diversity is employed by providing base stations with multiple antennas. The signals received at the antennas are typically combined using maximum ratio combining (MRC). Lee, supra for example, recognizes MRC as the best combining technique. In MRC, the received signals are combined based on the assumption that the interference closely approximates white Gaussian noise. An exemplary MRC scheme is shown in FIG. 2, where each signal branch (i.e., each received signal to be combined) is weighted by a selected weighting factor (.alpha..sub.1, .alpha..sub.2), and the signal branches are combined. MRC does not consider correlation between received signals, thereby enabling the received signals to be detected and equalized one at a time, and then combined by summing. MRC, since it assumes that the interference experienced by a signal closely approximates white Gaussian noise, has certain performance limitations when the interference does not closely approximate white Gaussian noise.
Alternatively, an improved method of combining received signals in a system with antenna diversity is known as interference rejection combining (IRC). IRC assumes that the received signals include both white Gaussian noise and signals from other transmitters (e.g., other mobile stations in other cells). Generally speaking, a receiver incorporating IRC produces received signal samples for each antenna (using, e.g., log-polar signal processing), estimates channel taps for each antenna, estimates impairment correlation properties (e.g., co-channel interference), forms branch metrics from the received signal samples, channel tap estimates, and impairment correlation estimates, and estimates the transmitted information sequence using the branch metrics (using, e.g., the Viterbi algorithm). The receiver estimates impairment correlation properties by estimating the correlated noise between signal branches when a training sequence (such as is contained in a typical GSM burst) is received. This estimated covariance is used by the receiver during the demodulation process. IRC is described in significant detail in the copending, commonly-assigned application Ser. No. 08/284,775 entitled "Method and Apparatus for Interference Rejection Combining in Multi-Antenna Digital Cellular Communications Systems", filed on Aug. 2, 1994 and copending, commonly assigned application Ser. No. 08/634,719 entitled "Method and Apparatus for Interference Rejection with Different Beams, Polarizations, and Phase References", filed on Apr. 19, 1996. These applications are hereby incorporated by reference in their entirety. The latter patent application discloses that IRC performance can be improved if the impairment correlation properties are scalar impairment correlation properties and the branch metrics are scalar branch metrics.
IRC is very efficient in rejecting interference from mobile stations from neighboring cells which transmit at the same frequency as the transmitted signal of interest (i.e., co-channel interference), particularly when an interfering burst is synchronized with the carrier burst (i.e., the transmitted signal of interest). IRC also reduces the effects of adjacent channel interference. Unfortunately, IRC is complex and requires a relatively large amount of computer processing resources. Further, there are some cases where IRC does not provide optimum performance.
It would be desirable to improve the performance of a communication system employing antenna diversity. More particularly, it would be desirable to improve known methods of diversity combining.